Epitaxy, the technique of depositing an oriented single crystalline layer on a crystalline substrate, can under certain conditions be used to deposit high quality layers on substrates having different physical properties from those of the epitaxial layer. Though widely and successfully used, epitaxial growth techniques are severely limited by substrate properties. Lattice mismatch between the substrate and the epitaxially grown layers in particular imposes one of the most severe restrictions. The mismatch of concern here is the difference between the substrate and epitaxial material lattice constants. The lattice constants of a crystalline material are the characteristic dimensions of a conventional unit cell determined by the crystal structure and spacing between atoms in the material. For example, a 0.3 percent or greater lattice mismatch between the substrate and epitaxial material will induce stress in the grown layer causing defects in the crystalline structure. Such defects quickly degrade the optical and electronic properties and will accordingly render the devices fabricated from the epitaxially grown layers unusable for optical, optoelectronic or electronic purposes.
Structures having different materials for the epitaxial grown layer and the substrate are referred to as heteroepitaxial structures, while structures which are comprised of the same epitaxial layer and substrate material are known as homoepitaxial structures. Heteroepitaxial structures can be used for many applications which do not have suitable homoepitaxial analogues. However, entire classes of devices that require unique heteroepitaxial structures can not be fabricated because of unacceptably large lattice mismatch in the heteroepitaxial system leading to unacceptable crystal defects. This situation exists in spite of various schemes that have been attempted to circumvent heteroepitaxial lattice mismatch problems. These include the use of superlattices (alternating thin layers of dissimilar materials) or thick strain grading (i.e., lattice-constant grading) layers intended to prevent development of crystal defects in the device active layers. Many applications-specific devices can not be made using these techniques because of an inability to incorporate the required material physical properties in the defect suppression structures.
Substrate physical properties, such as light absorption, and electrical and thermal conductivity, impose additional applications limitations on fabricated devices irrespective of whether they are fabricated using homoepitaxial or heteroepitaxial structures. As an example, optical absorption in substrate materials attenuates the outputs from light emitting diodes and lasers thereby limiting their operating efficiencies. Such light transmission limitations in substrates can also limit efficiencies and pose additional operation limitations for optical detectors, solar cells and other optoelectronic devices.
Many electronic applications for devices fabricated with epitaxially grown layers, particularly solid-state microwave and high-voltage devices, require electrically non-conducting substrates. Materials with large band gaps (the characteristic energy separating the valence and conduction bands in solid-state materials) are needed to meet this substrate requirement. However, substrate materials that have the appropriate lattice constant for the epitaxial system of interest may not have sufficiently large band gaps to be semi-insulating at device operating temperatures. Accordingly such materials can not be used for these applications, thereby, additionally restricting the types of devices that could potentially be made using epitaxial systems.
Another limitation is imposed by the substrate material thermal conductivity. In particular, high-power devices can have maximum power limitations imposed because of the inability to dissipate generated heat. Operation above the maximum power limits will excessively increase the device temperatures and accordingly decrease device performance and lifetime. Since device heat-sinking capabilities are usually controlled primarily by the thermal conductivity of the substrate, the maximum power at which devices can be operated is limited by the substrate used to support epitaxial growth.
While lattice mismatch in heteroepitaxial systems leads to defect generation in relatively thick epitaxially grown layers, thin strained epitaxial layers can be grown without crystal defects even when grown on grossly lattice mismatched substrates. Avoidance of crystal defects is achieved by imposing a thickness limitation on the epitaxial layers. By limiting the epitaxial layer thickness, the resulting stress due to the lattice mismatch is accommodated elastically through distortion of the epitaxial layer lattice constant. Such thin epitaxial layers which exhibit lattice constant adjustment through crystal lattice deformation are referred to as pseudomorphic layers. Crystal defects begin to occur in such layers as their thicknesses increase to the point where the total elastic strain energy exceeds the energy required for defect formation. Initiation of crystal defect formation effectively defines the maximum pseudomorphic layer thickness known as the pseudomorphic limit. This pseudomorphic limit is dependent upon the material properties of the epitaxial grown layer and the lattice mismatch between this material and the substrate. Depending upon the application many devices can not be made from pseudomorphic layers because the thicknesses as restricted by the pseudomorphic limit are too small.